1. Field of the Invention
The present invention relates to photodetection and, more particularly a photodetector of a waveguide type, a surface incidence type or the like suitable for use in optical integration circuitry and an optical communication system including such a photodetector. The photodetector is arranged to sweep out unwanted carriers or holes and may be used to receive a signal or light in an optical communication or similar system.
2. Related Background Art
FIG. 1 shows a prior art waveguide type photodetector. In FIG. 1, an undoped GaAs buffer layer 402, an undoped Al.sub.x Ga.sub.1-x As first clad layer 403, an undoped Al.sub.y Ga.sub.1-y As waveguide layer 404 having a thickness of several .mu.m and a undoped Al.sub.x Ga.sub.1-x As second clad layer 405 are successively layered on a semi-insulating GaAs substrate 400. Further, an active or light absorbing layer 406 having a thickness of 0.2 .mu.m to 0.5 .mu.m is layered, and a source electrode 411, a gate electrode 412 and a drain electrode 413 are deposited thereon. This photodetector operates in a biased state, and a light to be detected is propagated through the waveguide 404. As a result, a photocurrent flows between the drain electrode 413 and the source electrode 411, and the light is detected.
In the prior art photodetector, however, carriers (electrons and holes) generated by a light absorbed in regions other than the absorbing layer 406 (i.e., the waveguide layer 404, the buffer layer 402, the clad layers 403 and 405, the substrate 400 and the like) are not swept out and are likely to be stored therein and extinguished after a certain time. The response characteristic of the photodetector is adversely affected by the unwanted stored carriers that flow into a light detecting portion thereof within that extinction time (a recombination time of carriers). Holes, especially, present a difficult problem because their mobility is much smaller than that of electrons. Further, an electrostatic potential is caused by the stored carriers, and a potential at the side of the substrate 400 fluctuates. As a result, the confinement of carriers in the absorbing layer 406 is lowered, and the light-current characteristic of the photodetector becomes unstable. Hence, the detection sensitivity is likely to degrade.